Terrestrial cellular communication systems are well known in the art and a number of standards exist which define different signaling protocols for them, such as the GSM standard and the PCN standard. Details of these standards are given for example in “Mobile Radio Technology” by Gordon White, Butterworth Heinemann, 1994. The GSM and PCN systems are digital communication systems and allow paging and data communications in addition to voice communications. In the GSM system, a short message service (SMS) is provided for transmitting short messages to mobile terminals.
Mobile satellite communication systems have also been proposed in which radio frequency links are provided by a constellation of satellites. These satellite communication systems provide much greater coverage than terrestrial cellular systems. One example of such a system is the ICO™ system, aspects of which are described in patent publications WO 95/28747, WO 96/03814 and GB 2,295,296A. Other examples include the Inmarsat™ satellite system as described in “Satellite Communications: Principles and Applications” by Calcutt and Tetley, published 1994 by Edward Arnold, the Iridium™ satellite cellular system, described for example in EP-A-0365885, and the Odyssey™ system described for example in EP-A-0510789, EP-A-0575678 and EP-A-0648027.
However, in both terrestrial and satellite mobile communication systems, it is not always possible to communicate with a mobile user, since the received signal strength at the mobile user terminal may be severely attenuated, for example by blockage or multi-path fading. Furthermore, the gain of the user terminal may be low, for example because the user terminal antenna is retracted or not deployed in the optimum position.
A satellite paging system is disclosed in U.S. Pat. No. 5,392,451 in which, if a call receiver does not acknowledge receipt of a paging message, the paging signal is repeated with a higher transmission power.
A paging system is disclosed in W096/08941 in which, if a mobile telephone does not respond to a paging signal, the paging signal is sent on a different channel, with more error correction or higher power if necessary.
However, the transmission power of a satellite is limited by the power supply available, such as the output power of solar cells of the satellite or by the battery power of the satellite during an eclipse.